Modified plasma torch design for introducing sample air into inductively coupled plasma

ABSTRACT

A plasma torch for reliable analysis of airborne particulate matter permits real-time monitoring of airborne metal pollutants in flue gases from furnaces and incinerators. The torch injects sample air into argon plasma and has an outer tube to confine plasma gas for generating a plasma fireball. An intermediate tube has an outwardly flared portion concentrically disposed within the outer tube to form an outer annulus for feeding plasma gas to the fireball. The intermediate tube also has an injector sheath tube joined at its base to the base of the flared portion and concentrically disposed within the flared portion. The injector sheath tube is parallel to the outer tube. An inner capillary injector tube injects sample air into the plasma fireball. The inner capillary injector tube is concentrically disposed within the injector sheath tube to form an inner annulus that directs annular flow of auxiliary gas to alter the surface of the central region of the plasma fireball and channels the sample air to inject it through the surface and into the plasma fireball. The annular flow of auxiliary gas assures injection of the sample air at velocities which preserve residence time of the sample air to effect efficient vaporization and excitation of the metals.

STATEMENT OF GOVERNMENT INTEREST

The invention described herein may be manufactured and used by or forthe Government of the United States of America for governmental purposeswithout the payment of any royalties thereon or therefor.

CROSS REFERENCE TO RELATED APPLICATIONS

This is a continuation in part of U.S. patent applications entitled"Correction of Spectral Interferences Arising from CN Emission inContinuous Air Monitoring Using Inductively Coupled Plasma AtomicEmission Spectrometry" by Michael Seltzer. U.S. Patent and TrademarkOffice Ser. No. 08/932,023, Navy Case No. 77871, filed Sep. 17, 1997 nowpending and "Method and Apparatus for Automated Isokinetic Sampling ofCombustor Flue Gases for Continuous Monitoring of Hazardous MetalEmissions" by Michael Seltzer, U.S. Patent and Trademark Office Ser. No.08/932,401, Navy Case No. 78564, filed Sep. 17, 1997 now allowed and"Sampling Interface for Continuous Monitoring of Emissions" by MichaelSeltzer, U.S. Patent and Trademark Office Ser. No. 08/932,233, Navy CaseNo. 78274, filed Sep. 17, 1997 now U.S. Pat. No. 5,834,656 incorporatesall references and information thereof by reference herein.

BACKGROUND OF THE INVENTION

This invention relates to plasma torches used for spectrochemicalanalysis. In particular, this invention relates to a plasma torchdesigned to analyze samples of air in an inductively coupled plasma.

Plasma torches, since their inception in the early 1970s, have beendesigned expressly for use with plasma gases of homogeneous composition.This is in contrast to the invention to be described hereinbelow whichinvolves the introduction of sample air into argon plasma. Consequently,the design of these prior art torches has been optimized for homogeneousgases, primarily, because the prior art has not recognized the need tointroduce air into argon plasma, see, for example, U.S. Reissue Pat. RE29,304. Since argon inductively coupled plasmas are used exclusively forthe elemental analysis of liquid samples such as water, and maximumsensitivity is often required, it has been found that the addition ofother gases typically degrades analytical performance. Consequently, theintroduction of air into argon plasmas has been avoided and the absenceof this requirement until now has left the problems associated with airintroduction unsolved.

Referring to FIG. 1, typical prior art plasma torch T consists of threeconcentric quartz tubes that are fused or otherwise held together usingappropriate hardware. These tubes are commonly called outer tube OT,tulip-shaped intermediate tube IMT and central injector tube CIJT. Threedistinct argon streams flow through these tubes. Typically, throughnarrow annulus NA formed between outer tube OT and tulip-shapedintermediate tube IMT, plasma argon PA flows at rates of 15-20 litersper minute. In the large annulus LA located between intermediate tubeIMT and central injector tube CIJT, auxiliary argon AXA typically flowsat 0-2 liters per minute. Carrier or aerosol argon CA flows at 0.5-1liter per minute through central injector tube CIJT.

The narrow annulus NA in this classic torch geometry that was formedbetween tulip-shaped intermediate tube IMT and outer tube OT adequatelyprovided a suitable flow of plasma gas PA. However, the velocity ofauxiliary argon AXA through the area inside of the "tulip" has beenfound to be insufficient to promote satisfactory injection of sample airwith carrier aerosol CA into plasma fireball PF and produce accurateresults or avoid damaging the tubes. The state-of-the-art torches havehad great difficulty when air flow of approximately 0.5 liters perminute is added to the flow of argon carrier CA through carrier injectortube CIJT since some portion CAa of carrier CA would inadvertently flowaround the fireball. The geometry of standard plasma torches makes airinjection difficult to achieve under analytically-favorable conditions,and failure to achieve air injection leads to accelerated aging of thesetorches.

Thus, in accordance with this inventive concept, a need has beenrecognized in the state of the art for plasma torches that providereliable analyses of airborne samples.

SUMMARY OF THE INVENTION

The present invention is directed to providing a plasma torch forairborne particulate matter. The plasma torch has an outer tube toconfine plasma gas for generating a plasma fireball. An intermediatetube has an outwardly flared portion concentrically disposed within theouter tube to form an outer annulus for feeding the plasma gas to theplasma fireball. The intermediate tube also has an injector sheath tubejoined at its base to the base of the flared portion and concentricallydisposed within it. The injector sheath tube is parallel to the outertube. An inner capillary tube injects sample air into the plasmafireball and is concentrically disposed within the injector sheath tubeto form an inner annulus. The inner annulus directs the annular flow ofan auxiliary gas such as argon plasma gas to the central region of theplasma fireball and channels the sample air into the plasma fireball.

An object is to provide a plasma torch for analysis of elements.

Another object is to provide a plasma torch for analysis of airborneparticulate matter.

Another object is to provide a plasma torch for real-time monitoring ofairborne metal pollutants.

Another object is to provide a plasma torch for real-time monitoring ofairborne metal pollutants in flue gases from furnaces and incineratorsto promote pollution prevention and to ensure continuous compliance withenvironmental regulatory requirements.

Still another object is to provide to provide a plasma torch thatinjects sample air into argon plasma for detecting airborne metals.

Still another object is to provide to provide a plasma torch that avoidsthe problems associated with analysis of a sample air stream.

Another object is to provide a plasma torch that injects sample air intoargon plasma at velocities that are favorable for sensitive detection ofairborne metals.

An object of the invention is to provide a plasma torch that allowssufficient residence time for airborne metals in the plasma forefficient vaporization and excitation of the metals.

Another object is to provide a plasma torch having an injector sheathtube to focus auxiliary argon flow onto the central region of the plasmafireball to alter the shape of the fireball's surface to facilitatepenetration of the sample air stream into the fireball.

Another object is to provide an argon torch permitting efficientintroduction of sample air into argon plasma to provide desiredanalytical performance while protecting the quartz torch from rapidthermal degradation caused by air transferring from the plasma.

These and other objects of the invention will become more readilyapparent from the ensuing specification when taken in conjunction withthe appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional side view of a classical plasma torch of theprior art.

FIG. 2 is a cross-sectional side view of the plasma torch in accordancewith this invention.

FIG. 3 is a cross-sectional end view of the plasma torch of thisinvention taken generally along lines 3--3 in FIG. 2 and showingexaggerated cross-sectional dimensions with respect to the otherfigures.

FIG. 4 is a cross-sectional side view of the plasma torch in accordancewith this invention showing injection of the sample air into the plasmafireball through the altered surface of the fireball.

DESCRIPTION OF THE PREFERRED EMBODIMENT

A recent development has recently focussed attention on instrumentationand methodology for real-time monitoring of airborne metal pollutants,see U.S. Pat. No. 5,596,405. A sample stream of air is injected directlyinto an inductively coupled argon plasma and airborne metals arevaporized and excited resulting in emission of characteristicwavelengths of light. Inductively coupled plasma is sustained by flowingargon through a quartz plasma torch positioned axially within a helical,radio frequency induction coil. Injection of air into the argon plasmahis been accomplished with some difficulty and somewhat satisfactoryresults have been obtained.

However, in accordance with this invention, two serious problems havebeen noted that require an examination of the process of air injectioninto argon plasma an optimization of this process. Because ofdifferences in thermal and electrical properties of argon and air(nitrogen, oxygen), and certain hydrodynamic properties of the argonplasma, injection of the sample air stream cannot be achieved unless thevelocity of the air stream is sufficiently high to overcome a phenomenonknown as magnetohydrodynamic thrust, see J. D. Chase, "Theoretical andExperimental Investigations of Pressure and Flow on Induction Plasmas,"J. Appl. Physics, 42 (1971) 4870-4879. (When a stream of argon isinjected into the argon plasma in a conventional torch, adequatepenetration in the plasma fireball is achieved with considerable easeand at much lower velocities.) Unfortunately, at the velocities requiredto achieve penetration of the argon plasma with an air stream, airbornemetals entrained in the sample air stream are not given sufficientresidence time in the plasma to undergo adequate vaporization andexcitation to promote sensitive detection.

Consequently, in accordance with this invention, it has been recognizedthat the potential sensitivity of this approach has not been entirelyrealized. Secondly, using a conventional plasma torch, the air is notinjected in its entirety through the argon plasma. Instead, most of itis forced, or deflected, to flow around the outside of the argon plasma.Air has much higher thermal conductivity than pure argon Thus,considerable heat transfer occurs between the hot argon plasma and thequartz torch used to sustain the plasma. Under normal circumstances,when pure argon plasma is operated, little heat transfer to quartz tubesoccurs, and torches are spared thermal degradation resulting in hundredsof hours of useful operation. In accordance with this invention, it hasbeen discovered that where a sample air stream is introduced into argonplasma there is overheating of a conventional plasma torch due toinadequate penetration of the argon plasma by the air stream. Thisresults not only in pronounced degradation of analytical performance butcreates de-vitrification of the quartz and accelerated aging of theconventional torch resulting in useful operating lifetimes of less than20 hours.

Therefore, this invention provides a plasma torch design that makes itmore amenable to the introduction of a sample air stream. The inventionpromotes improved penetration of the air stream into the argon plasmaresulting in enhanced sensitivity for airborne metal detection andprotection of the quartz plasma torch from thermal degradation. As aconsequence, the lifetime of the torch of this invention is extended atleast tenfold resulting a direct cost savings.

Referring to FIGS. 2, 3, and 4 of the drawings, plasma torch 10 hasouter tube 11 provided with fitting 12 for introducing argon plasma gas13. Gas inlet fitting 12 is mounted tangentially on tube 11 to induce atangential flow of plasma gas 13 within outer tube 11. This tangentialflow component of the plasma gas has been found to be advantageous forthis plasma torch design. Typically, outer tube 11 has an outer diameterof about 20 mm and an inner diameter of about 18 mm.

An intermediate tube 15 is concentrically disposed within tube 11 andhas concentrically disposed outwardly flared portion, or "tulip" 16 thatdefines outer annulus 17 between it and the inner surface of outer tube11. Typical dimensions for the outer diameter of portion 16 are about 17mm and inner diameter of about 15 mm. An annular flow of plasma gas 13ais fed through annulus 17 to feed and sustain plasma fireball 50 andtail flame 50a. The plasma is inductively coupled to an induction coil,not shown, and the coil is driven by a suitable radio frequency energysource to sustain the plasma fireball according to procedures well knownin the art.

Intermediate tube 15 is also provided with injector sheath tube 18.Injector sheath tube 18 is joined at its base 18a to base 16a of flaredportion 16. The bases can be joined in a variety of ways either duringformation by the glass blower or the bases can be fused or appropriatelyfitted later. Whatever fabrication technique is selected, injectorsheath tube 18 is concentrically disposed within flared portion 16 andis parallel with outer tube 11. Injector sheath tube 18 has an outerdiameter of about 9 mm and an inner diameter of 7 mm.

Inner capillary injector tube 20 is concentrically disposed withininjector sheath tube 18 and is provided with gas inlet fitting 21 toreceive sample air 22 in tube 20 and inject it as sample air stream 22ainto fireball 50. Sample air 22 contains mixed air (nitrogen, oxygen),argon and air-entrained materials, such as metals or other constituentsof interest. The air and air entrained materials in sample air 22 can betaken from flue gases containing airborne metal pollutants from furnacesand incinerators or can be air from an area containing hazardouschemicals, etc. Predetermined amounts of argon are mixed with the otherconstituents to facilitate injection in plasma fireball 50.

Inner capillary injector tube 20 has an outer diameter of about 4 mm andan inner diameter of about 1.5 mm and receives sample air 22 and injectssample air stream 22a through end 20a. The outer surface of innercapillary tube 20 forms a 1.5 mm thick inner annulus 23 between theinside of injector sheath tube 18 and it, to pass and direct an annularflow 26 of auxiliary gas 25 to central region 51 of plasma fireball 50.

Auxiliary gas 25 is argon introduced into intermediate tube 15 via gasinlet fitting 27. Fitting 27 is mounted perpendicularly on intermediatetube 15 to direct the flow of auxiliary gas 25 toward the longitudinalaxis of intermediate tube 15. Locating fitting in such a mannereliminates swirling or tangential motion of the auxiliary argon so thatit can more readily accomplish its intended function. Annular flow 26forms a concentric sheath flow 26a toward and onto central region 51.Flow 26a alters the shape of central region 51, allows penetration ofsample air into plasma fireball 50, and channels sample air 22a onto andinto the fireball.

Plasma torch 10 includes injector sheath tube 18 around inner capillaryinjector tube 20 to create a concentric `sheath flow` 26a of auxiliaryargon 25 around the injected sample air stream 22a. This capability is amarked difference from the prior art standard torch mentioned above thathas the flow of argon through the enlarged portion of the intermediatetube that surrounds the injector tube. Under operating conditions wherepure argon plasma is sustained, the arrangement of the prior art torchdoes facilitate penetration of aerosol into the plasma fireball bymodifying the bottom of the fireball.

However, the prior art approach is not nearly as effective where an airstream is to be injected into the fireball. To achieve partialeffectiveness in this prior art torch, auxiliary argon must flow throughthe intermediate tube at volume levels, or mass flow-rates approachingthose that are sufficient to extinguish the plasma.

Contrary to the prior art torch, plasma torch 10 additionally hasinjector sheath tube 18 that measures about 9 mm outer diameter andabout 7 mm inner diameter. Injector sheath tube 18 concentricallysurrounds inner capillary tube 20 (outer diameter about 4 mm) andextends at its end 18b to about 3 mm beyond end 20a of inner capillarytube 20. This arrangement creates a high velocity argon sheath flow 26aof auxiliary gas 25 that facilitates penetration of injected air stream22a into plasma fireball 50 and, simultaneously, prevents flow ofinjected sample air stream 22a around fireball 50. This helps minimizethermal degradation of the tulip of intermediate tube 16. Therefore, byreducing the cross-sectional area, or thickness, of inner annulus 23 toabout 1.5 mm, annular flow 26 of auxiliary argon 25 flows withdramatically increased flow velocity and also is more focussed in sheathflow 26a. The desired penetration of plasma fireball 50 is achievedwithout requiring mass flow rates, or volumes, that threaten toextinguish the plasma.

Thus, relatively high velocity argon sheath flow 26a of auxiliary argonessentially alters the surface of plasma fireball 50 therebyfacilitating penetration of injected sample air stream 22a. Injection ofair thereby is accomplished using flow velocities of the sample air thatallow favorable residence times for the sample air in the plasma. Thisresults in satisfactory analytical performance.

The constituents of plasma torch 10 are made from a variety ofappropriately sized sections of quartz tubing or any other workable,high refractory material (ceramics) capable of withstanding thetemperatures and energies associated with inductively coupled plasmas.Furthermore, the fabrication techniques for the novel structure ofplasma torch 10 are chosen from a number of widely used provenprocedures to produce a plasma torch that will give prolonged usefulservice.

Accordingly, plasma torch 10 can be fabricated by properly equipped,competent glassblowers. To create the sheath argon flow described above,a quartz tube (injector sheath tube 18) is located concentrically aroundinner capillary tube 20 as shown in FIGS. 2, 3, and 4. Injector sheathtube 18 can be added at the time of fabrication and should be done so ina manner that the original geometry of the standard torch is notchanged. The `tulip` shape of flared portion 16 of intermediate tube 15is preserved to ensure proper operation. Injector sheath tube 18 may beregarded as an extension of the `stem` of the `tulip`. The actualdiameter of injector sheath tube 18 may vary; but to ensure maximumeffectiveness, inner annulus 23 between injector sheath tube 18 andinner capillary injector tube 20 should not exceed 1.5 mm. As mentionedabove, if the tip of inner capillary injector tube 20 has an outerdiameter of 4 mm then the inner diameter of injector sheath tube 18should be no larger than 7 mm. Since the inner diameter of the `tulipstem` portion of intermediate tube 15 is typically 10 mm, a taperedtransition or interface 18c will be required to join injector sheathtube 18 to the `tulip stem`. This interface 18c is shown in the regionthat includes ends 16a and 18a of flared portion 16 and injector sheathtube 18, respectively. It is absolutely essential that all quartz tubesused in torch fabrication are aligned and made concentric in the finalsteps of manufacture.

As an alternative to permanently affixing injector sheath tube 18 tointermediate tube 15, a demountable arrangement can be configured inwhich the halves of a tapered ground joint can be fashioned on the endof injector sheath tube 18 and on the inside of the `tulip stem`,respectively. This would be similar to technology described in U.S. Pat.No. 4,739,147.

Irrespective which fabrication approach is selected, end 18b of injectorsheath tube 18 extends to the same level as end 16b of flared portion 16(tulip) of intermediate tube 15. End 20a of inner capillary injectortube 20 is recessed up to 3 mm back from the ends 18b and 16b ofinjector sheath tube 18 and flared portion 16, respectively.

Addition of injector sheath tube 20 allows focusing flow sheath 26a ofauxiliary argon flow 26, resulting in a concave indentation, or at thevery least, flattening of the argon plasma fireball 50. This greatlyfacilitates injection of sample air stream 22a into argon plasmafireball 50 in a manner that retains desired analytical properties ofthe argon plasma and, simultaneously, protects the components of torch10 from thermal damage caused by undesirable flow of air around fireball50.

Torch 10 fabricated in accordance with this invention is able to injectsample air 21 into argon plasma fireball 50 at velocities that arefavorable for sensitive detection of airborne metals. The airborneparticulate matter containing the metals thereby has sufficientresidence time in the plasma fireball to effect efficient vaporizationand excitation of the metals.

Prior to this invention analysis of air samples was difficult since theelectrical and thermal differences between plasma gas (argon) and samplegas (air) made the injection process difficult. The argon plasmafireball in prior art torches resisted penetration by the injected airstream and diverted most of it around the fireball. This resulted inpoor, if any, excitation of airborne metals and inadvertent damage tothe quartz torch tubing due to the relatively high thermal conductivityof air.

The present invention recognizes and addresses these problems of theprior art by introducing what may be the only significant modificationof the classic plasma torch design in years| The addition of injectorsheath tube 20 allows auxiliary argon flow 26 to be focused into anargon sheath flow 26a that impinges central region 51 of plasma fireball50 in a fashion that alters the shape of the fireball's surface andfacilitates penetration of sample air stream 22a into fireball 50. Thus,the invention permits injection of sample air into argon plasma fordetecting airborne metals in a revolutionary application of technology.This innovation permits efficient introduction of sample air into argonplasma while maintaining desired analytical performance and, at the sametime, protecting the quartz torch from rapid thermal degradation causedby the presence of air in the plasma.

Representative operational parameters of torch 10 are:

    ______________________________________    Function      Range         Optimum    ______________________________________    plasma gas 13 15-17 1/min. argon                                16 1/min. argon    Aux. gas flow 26                  0.7-0.9 1/min. argon                                0.85 1/min.    argon    Sample air stream 22a                  0.3-0.6 1/min.                                0.45 1/min.                  sample air    sample air                  plus          plus                  0.1-0.2 1/min. argon                                0.15 1/min. argon    ______________________________________

It is emphasized that the dimensions of injector sheath tube 18 andinner capillary injector tube 20 are critical since they define the sizeof inner annulus 23 formed between them. In accordance with thisinvention preferably torch 10 is fabricated with inner annulus 23 havinga thickness of not greater than 1.5 mm. This assures that torch 10creates argon sheath flow 26a to alter central surface 51 of argonplasma fireball 50 and to make it more amenable to penetration by sampleair stream 22a. In other words, injector sheath tube 18 in torch 10 isresponsible for the creation of the appropriate gas velocity that shapesa concave indentation or flattening of plasma fireball 50 to facilitateinjection of sample air stream 22a at velocities sufficiently low toretain sufficient sample residence time in plasma fireball 50. Incontradistinction, the prior art torch does not have injector sheathtube 18. This necessitates injection of sample air at much higher massflow velocities; however, at these higher mass flow velocities, sampleresidence times are too short for optimum analytical performance.Similarly, a prior art torch that does not have injector sheath tube 18may have to dramatically increase the auxiliary argon flow to attempt toimprove penetration of the plasma fireball by sample air; however, atthe high auxiliary argon flows required, the plasma becomes unstable andis easily extinguished.

In accordance with this invention, when the inner capillary injectortube 20 has a 4 mm outer diameter the injector sheath tube 18 shouldhave an inner diameter of 7 mm to create an annulus of 1.5 mm. Anauxiliary argon flow range of 0.7-0.9 liters per minute fed through thisannulus makes a focused sheath flow velocity range of 45-58 centimetersper second.

If the inner diameter of injector sheath tube 18 were to be increased, aproportional increase in auxiliary argon flow 26 would be required toprovide the same sheath flow velocity. However, care must be exercisedsince increased mass flow of argon may have deleterious effects onplasma fireball 50. This is because increased mass flows of auxiliaryargon may be high enough to lift or otherwise displace plasma fireball.This is a serious problem since the fireball may be lifted to the extentthat interaction with the radio frequency load coil that supplies energyto the plasma will be decreased and the plasma could be extinguished.

If, on the other hand, injector sheath tube 18 were to be decreased toan inner diameter smaller than 7 mm, a proportional decrease ofauxiliary argon flow would be required to provide the same sheath flowvelocity. In this case, the decreased mass flow of argon woulddefinitely have deleterious effects on the analytical performance of theplasma.

Therefore, torch 10 has inner capillary injector tube 20 with a 4 mmouter diameter and injector sheath tube 18 with an inner diameter of 7mm to provide the suitable gas velocities at the volumetric flow rateslisted above.

Although torch 10 has been described as being fabricated from materialswhich accommodate argon within described flow rates, it is to beunderstood that other torch configurations, gases, and flow rates couldbe selected having the teachings of this invention in mind. The exactconfiguration of torch 10 is decided by the requirements of the job athand. Therefore, it is to be understood that, having the teachings ofthis invention in hand, one skilled in the art to which this inventionpertains could configure the plasma torch of this invention differently,and still be within the scope of this inventive concept. This selectionof other known materials to meet other analysis requirements is wellwithin the purview of one skilled in the art in view of the teachings ofthis invention.

Therefore, it should be readily understood that many modifications andvariations of the present invention are possible within the purview ofthe claimed invention. It is to be understood that within the scope ofthe appended claims the invention may be practiced otherwise than asspecifically described.

I claim:
 1. An inductively coupled plasma torch for exciting a chemicalelement to emit electromagnetic radiation, said plasma torch having anouter tube, an inner tube coaxial therewith, an axially disposedcapillary tube, a first feed line for conveying argon plasma gas, asecond feed line for conveying argon cooling gas and a third feed linefor conveying a sample argon gas;said sample argon gas includes asuspended aerosol said aerosol containing chemical elements to beexcited; said plasma torch is mounted coaxially within an energizedinduction coil; said argon cooling gas is caused to flow through saidplasma torch; a plasma fireball is sustained within the confines of saidouter tube and above said inner tube; said argon sample gas isintroduced through said capillary tube penetrating said plasma fireballsuch that said aerosol traverses central portion of said plasma fireballto achieve maximum excitation; said argon plasma gas conveyed by saidfirst feed line and flowing through the wide space between said innertube and said capillary tube is not essential for sustaining said plasmafireball, but is useful for lifting said plasma fireball to preventmelting of said capillary tube and to prevent the formation of carbondeposits on said capillary tube, the improvement comprising, theaddition of an injector sheath tube mounted concentrically about saidcapillary tube and extending to a length such that the open end of saidinjector sheath tube is parallel with the open end of said capillarytube; opposite to said open end said injector sheath tube is joined tothe base of said inner tube such that the flow of said argon plasma isdirected through the narrow confines of the space between said injectorsheath tube and said capillary tube in such a manner as to concentratesaid flow of said argon plasma gas so that said flow deliberately makescontact with the surface of said plasma fireball at the point ofinjection of a sample gas.
 2. The improvement defined in claim 1 inwhich said flow of said argon plasma gas established by said injectorsheath tube alters the surface of said plasma fireball and facilitatesthe penetration of molecular sample gases such as air or flue gas intosaid argon plasma fireball.
 3. The improvement defined in claim 2 inwhich said flow of said argon plasma assures complete penetration ofsaid molecular sample gas introduced at optimum flow rates of about0.3-0.6 liters per minute which preserves residence time in said plasmafireball for said molecular sample gas to effect efficient vaporizationand excitation of entrained metals.
 4. The improvement defined in claim2 in which said molecular sample gas is mixed air (nitrogen, oxygen),argon and air-entrained metals.
 5. The improvement defined in claim 2 inwhich said argon plasma gas is at a flow rate of about 0.7-1.0 litersper minute.
 6. The improvement defined in claim 2 in which saidmolecular sample gas includes air or combustor flue gas.
 7. Theimprovement defined in claim 2 in which said injector sheath tube has aninner diameter of about 7 mm and said capillary tube has an outerdiameter of about 4 mm to create an annulus of about 1.5 mm.